The present invention relates to a golf ball having a plurality of dimples formed on a surface thereof, a core formed by vulcanizing a rubber composition, and a cover composed primarily of a polyurethane material.
Although an increase in distance on shots with a driver is an appealing feature to golfers, middle irons also have their appeal in golf play. As used herein, “middle irons” generally refers to the number five to seven irons—that is, irons capable of achieving a ball distance of about 130 to 170 yards.
It is desired that the distance traveled on shots with a middle iron be increased, and also that flight performance and other ball performance attributes, including durability to cracking on repeated impact and scuff resistance, be maintained. Recently, golf balls in which the cover material is a urethane material instead of the ionomers commonly employed until now have become conspicuous. Such urethane cover golf balls, in addition to achieving an excellent flight on driver shots, are also capable of improved controllability due to the increased spin rate on approach shots.
However, in conventional golf balls, the core, which accounts for most of the volume of the overall ball and is generally formed of a rubber composition, exerts a large influence on ball performance. Golf balls aimed at achieving synergistic effects between the core and the urethane cover include those described in, for example, JP-A 2007-152090, JP-A 2008-194470, JP-A 2008-194473 and JP-A 2004-121815.
JP-A 2007-152090 describes a two-piece solid golf ball with a urethane cover, which ball uses a core having a hardness profile limited to a Shore D value of 20 or below. However, the core cross-sectional hardness is not optimized, as a result of which the spin rate of the ball rises, shortening the distance traveled. In addition, this ball has a poor resistance to discoloration.
JP-A 2008-194470 describes a urethane cover golf ball in which the core has a relatively low surface hardness. However, in this golf ball, the cross-sectional hardness of the core is not optimized, as a result of which the spin rate of the ball rises, shortening the distance traveled. Moreover, the ball has a poor discoloration resistance.
JP-A 2008-194473 describes a golf ball having a core formed of a rubber composition which contains sulfur in the base rubber and has a high loading of unsaturated carboxylic acid or a metal salt thereof. However, this golf ball has a poor discoloration resistance.
JP-A 2004-121815 discloses a golf ball in which the base rubber contains a monophenol-type antioxidant and has a relatively small loading of organic peroxide.